Step 1 : First ask some one to multiply their Birth month by 31 and birth day by 12. Then ask them to some both...

Step 2 : Now ask the sum amount from them..

Step 3 : Now you've got the sum of M*31+D*12= Z ( Say ) M - Month D - Day Z - Total of above sum. - this is the value that you got. But you don't know about the M,D..

Step 4 : Now Do this algorithmic procedure...

[ Z ] / 12 = P + Q/12 -------- (01)
[ Q * 7 ] / 12 = R + S/12 -------- (02)
So Now S - is the Birth Month( that is M )
And P - 2*S - R = is Birth Day ( that is D )

Eg : My Birth Month = 4
My Birth Day = 17

So (4 * 31 + 17 * 12 = ) 328 is the value that I got.

Now do the algorithm to fine the M, D from the above value... ( Here the problem is you have two unknown values but one equation )

328 / 12 = 27 + 4/12
So P = 27;
Q = 4;


Now Q * 7 / 12 = 4 * 7 / 12 = 2 + 4/12
So R = 2
S = 4

From this P, Q, R, S you can conclude that
Birth Month = 4 [ from C ]
Birth Day = 17 [ Because P - 2*S - R = (27 - 4*2 - 2) = 17 ]


So you can find this algorithmic problem and confuse you friends...